Are you asking about a marker light for your cylinder?
If so, I have used a couple:
- Princeton Tec has a nice marker light they call the Eco Flare and I love the lifetime replacement warranty (10 year international) Princeton Tec has on their lights
- Glo-Toob LED light is much smaller (similar to a AA battery) and allows you to change the color of the LED so that your buddy can identify you more easily
- last trip I tried the Bigblue easy clip. Very small and bright, and again allows you to change the color. Unlike the other lights, this one is negatively buoyant, so it doesn’t float above the first stage. I liked its size, but need to play with placement a little bit more to make it easier to see from more directions (or clip a piece of foam to it to make it positively buoyant.